The original idea of using a quadrupole mass analyzer and a quadrupole ion trap for mass analysis was first disclosed by W. Paul and H. Steinwedel in U.S. Pat. No. 2,939,952. In general, two different electrode structures are used in quadrupole ion trap mass spectrometry; that is, the linear quadrupole ion trap structure and the 3D rotationally symmetric quadrupole ion trap structure, illustrated in FIGS. 1a and 1b respectively of the accompanying drawings. Referring to FIG. 1a, the linear quadrupole ion trap structure includes a pair of x-electrodes 1, a pair of y-electrodes 2, an ion entrance plate 3 and an ion exit plate 4. Both plates 3,4 can be used to set a potential barrier to prevent ions from escaping. Referring to FIG. 1b, the quadrupole ion trap structure includes a ring electrode 1, and end cap electrodes 2,3, there being a central hole 4 in end cap electrode 2. To make these structures function as mass analyzers, a voltage having a periodic variation as a function of time needs to be applied across the electrodes. U.S. Pat. No. 2,939,952 teaches a method of generating a sinusoidal high frequency voltage combined with a DC voltage to achieve this periodic voltage. Upon application of such a voltage a quadrupole electric field that drives the ions' motion is set up. The theory of ion motion based on the solution of Mathieu's equation was established. This theory has been widely used by others in later developments of quadrupole mass spectrometry and introduced in the related text book “Quadrupole Storage Mass Spectrometry” by E. March, R. J. Hughes, Wiley—Interscience Publication where the sinusoidal high frequency voltage is usually referred to as a radio frequency (RF) voltage.
There were many technical advances of ion trap mass spectrometry in the 1980's. Among them, operation in mass selective instability mode disclosed in U.S. Pat. No. 4,540,884 and use of mass selective resonance ejection disclosed in U.S. Pat. No. 4,736,101 led to significant improvements in the performance of a quadrupole ion trap enabling the device to carry out fast and high resolution mass analysis and tandem mass analysis.
Different methods of detection such as Fourier transform of image current disclosed in U.S. Pat. No. 5,629,186 were also developed later. These developments have brought about tremendous applications in mass spectrometry and in the combination of mass spectrometry with other widely used instrumentation.
Because, fundamentally, this technology is based on ion motion in the superimposed RF and DC quadrupole electric fields, or in some cases in a pure RF electric field, all applications need an RF power source to supply RF voltage to the quadrupole devices. Conventionally, a RF power supply comprises a driving electric circuit and a resonating network which includes the quadrupole ion optical device as a load. The resonant frequency of the network is normally fixed or has a small number of fixed values. To achieve mass scanning or mass selection, the output voltage of the RF power supply must be able to ramp up and down precisely according to the desired scheme, the amplitude of the RF voltage being proportion to mass-to-charge ratio when the RF frequency is fixed. A high RF voltage is necessary for high mass analysis. Also, sometimes an undesirable shift in the resonance position of the network caused by a change in output voltage needs to be corrected. These factors have resulted in increased costs and complexity of instruments.
A paper entitled “Frequency Scan for the Analysis of High Mass Ions Generated by Matrix-assisted Laser Desorption/Ionization in a Paul Trap” by U. P. Schlunegger et al, Rapid. Commun. Mass. Spectrom. 13, 1792–1796 (1999) discloses use of a frequency scanning technique instead of a voltage scanning technique to improve the mass scanning range of a quadrupole ion trap of a MALDI ion trap spectrometer. The described technique is particularly suitable for trapping and analysing biomolecular ions which have high mass-to-charge ratio. A waveform generator and a power amplifier were used to provide the frequency-variable sine wave voltage. This voltage output is limited by the power consumption of the amplifier which is basically an analogue circuit and has to work in a linear state. Therefore, when a higher trapping RF voltage is needed, it is difficult to reduce the power consumption, and so the machine size and production cost with this configuration.
It is in fact not necessary to use a sinusoidal RF voltage to drive a quadrupole ion trap or a quadruople mass analyser as stated by W. Paul etc in their original disclosure. E. P. Sheretov et al in their paper “Basis of the theory of quadrupole mass spectrometers during pulse feeding” Zh. V. I Terent'ev, Tech. Fiz (1972), 42(5) 953–962 have given some detailed discussion on ion behaviour in the quadrupole mass spectrometer upon applying voltage pulses. GB 1346393 has even disclosed methods of driving a quadrupole mass filter with a rectangular or trapezoidal wave voltage. However, the real advantage of rectangular wave driving is associated with digital frequency scanning and timing control. This was not revealed by the previous art. The particular method combined with the rectangular wave driving of the quadrupole ion trap to achieve high performance MS and MSn has not yet been provided.
The method of this invention utilizes a time-varying rectangular wave voltage applied to a quadrupole ion trap device for ion trapping, selection, and/or mass analyzing.